High pressure shop air, or “HP air” is typically pressurized in a compressor to about 125 psig for storage in a tank for various shop uses. Storage tank pressure typically ranges from a high of 125 psig to a low of 115 psig, at which point the compressor comes on again to raise the pressure back to 125 psig. HP air from the tank is piped throughout the plant as Motive air for pneumatic equipment, or as pressurized air for purposes such as spraying or cleaning. While “high pressure” has to be high enough to meet all of these various requirements, some equipment operates at pressures lower than the “high pressure” level. For such lower pressure applications, a pressure reducing valve is required upstream of the equipment to reduce the pressure input to such equipment. A pressure reducing valve is a modulating orifice which allows high pressure air to expand to a lower pressure.
The problem with prior art systems as just described is that HP air is wasted by putting it through a reducing valve, wasting also the energy used to compress the HP air in the first place.
Factories often use many and various types of air driven equipment with varying requirements of air pressure and flow rate. The compressor and associated air tank are sized to meet the total pressure and volume requirements of all the pneumatic equipment in the factory. Typical pneumatic equipment takes in “Motive” air, and divides it into “Control” and “Process” air. Control air controls equipment operation. Process air does the work. In an air-operated diaphragm pump, Control air operates air direction control (DC) valves which direct Process air against the pump diaphragm, thereby to pump fluid (liquid or gas). Control air and Process air combined are then both exhausted from the pump to atmosphere.
It is an industry rule of thumb that a 2 psi change of output pressure corresponds with a 1% change of horsepower required to generate it. Thus, a pressure reduction as described above, from 125 psig to, say, 75 psig (a 50 psi reduction) represents a waste of 25% of the energy required to generate it. In other words, 25% less horsepower is required to compress air to 75 psig than is required to compress air to 125 psig. Another industry standard, relevant here, is that one horsepower is required to compress 4 cfm to 100 psig (i.e. 4 cfm/hp).
In addition to the term “high pressure” (HP), the term “low pressure” (LP) is used herein, abbreviated as indicated.
Pumps of the type described here are disclosed in U.S. Pat. No. 4,247,264 to Wilden.